1. Field of the Invention
This invention relates to treatment of obesity and related diseases, such as hyperinsulinmia, hyperglycermia, hypertension, cardiovascular diseases, muscular dystrophy and infertility. More particularly, the invention relates to methods of treating obesity and non-insulin-dependent (type II) diabetes mellitus (NIDDM) by specifically targeting the genes and gene products of cathepsins.
2. Description of Related Art
Obesity is the most important nutritional disorder in the western world, with the estimates of its prevalence ranging from 30% to 50% within the middle-aged population. Obesity is usually defined as a body weight more than 20% in excess of the ideal body weight. Severe obesity can be a chronic disease that affects an increasingly large number of people and requires long-term treatment to promote and sustain weight loss.
Obesity is caused by accumulation of excess adipose tissue containing fat cells, or adipocytes, most predominately under the skin, in the abdominal cavity, in skeletal muscle, round the blood vessels, and in mammary gland. The adipose tissue of a normal 70-kg man contains about 15 kg of fat.
Adipocytes are developed from fibroblast-like cells, both during normal mammalian development and in various pathological circumstances for example, in muscular dystrophy, where the muscular cells die and are gradually replaced by fatty connective tissue. Adipocytes differentiation begins with the production of specific enzymes, followed by the accumulation of fat droplets, which then coalesce and enlarge until the cell is hugely distended, with only a thin rim of cytoplasm around the mass of lipid. Sul (1989) Curr. Opin. Cell Biol. 1:1116-1121.
Various factors influence the process of adipocyte differentiation. One of the factors was identified as growth hormone, a protein normally secreted into the bloodstream by the pituitary gland. But growth hormone is not the only secreted signaling molecule that regulates adipocyte development. Adipocyte precursors (preadipocites) that have been stimulated by growth hormone become sensitive to insulinlike growth fact-1 (IGF-1), which stimulates the proliferation of the differentiating fat cells. Recently, it has been found that integration of leptin, an adipocyte-derived hormone, in hypothalamic networks results in activation of peripheral metabolic pathways that control energy build-up and expenditure. Plasma leptin levels correlate with fat stores and respond to changes in energy balance. It was initially proposed that leptin serves a primary role as an anti-obesity hormone, and this role is commonly thwarted by leptin resistance. Ahima and Flier (2000) Annu. Rev. Physiol. 62:413-437.
Currently the medications most often used in the management of obesity are commonly known as xe2x80x9cappetite suppressantxe2x80x9d medications. Appetite suppressant medications promote weight loss by decreasing appetite or increasing the feeling of being full. These medication decrease appetite by increasing serotonin or catecholaminexe2x80x94two brain chemicals that affect mood and appetite. Examples of prescription appetite suppressant medications include dexfenfluramine (REDUX(copyright)), diethylpropion (TENUATE(copyright), TENUATE DOSPAN(copyright)), fenfluramine (PONDIMIN(copyright)), mazindol (SANOREX(copyright), MAZANOR(copyright)), phendimetrazine (BONTRIL(copyright), PLEGINE(copyright), PRELU-2(copyright), X-TROZINE(copyright)), phentermine (ADIPEX-P(copyright), FASTIN(copyright), IONAMIN(copyright), OBY-TRIM(copyright)), and sibutramien (MERIDIA(copyright)).
There are some potential side effects associated with long term use of these medications. For example, two FDA-approved appetite suppressant medications that affect serotonin release and reuptake have been withdrawn from the market (fenfluramine and dexfenfluramine). Medications that affect catecholamine levels (such as phentermine, diethylpropion, and mazindol) may cause symptoms of sleeplessness, nervousness, and euphoria. The primary known side effects of concern with sibutramine are elevation in blood pressure and pulse, which are usually small, but may be significant for people with poorly controlled high blood pressure, heart disease, irregular heart beat, or history of stroke.
Obesity is associated with an increased risk for cardiovascular diseases, diabetes, stroke, muscular dystrophy and infertility. In particular, obesity can evolve to type II diabetes in successive phases. Clinically, these phase can be characterized as normal glucose tolerance, impaired glucose tolerance, hyperinsulinemic diabetes, and hypoinsulinemic diabetes. Such a progressive impairment of glucose storage correlates with a rise in basal glycemia.
Generally, there are two major forms of diabetes mellitus: insulin-dependent (type I) and noninsulin-dependent diabetes mellitus (type-II). Type I diabetes, also called juvenile-onset diabetes mellitus, most often strikes suddenly in childhood. In contrast, type II diabetes, also called maturity-onset diabetes mellitus, usually develops rather gradually after the age of 40.
The polypeptide hormone insulin acts mainly on muscle, liver, and adipose tissue cells to stimulate the synthesis of glycogen, fats, and proteins while inhibiting the breakdown of these metabolic fuels. In addition, insulin stimulates the uptake of glucose by most cells, with the notable exception of brain and liver cells. Together with glucagon, which has largely opposite effects, insulin acts to maintain the proper level of blood glucose.
In diabetes, insulin either is not secreted in sufficient amounts or does not efficiently stimulate its target cells. As a consequence, blood glucose levels become so elevated that the glucose xe2x80x9cspills overxe2x80x9d into the urine, providing and convenient diagnostic test for the disease. Yet, despite of these high blood glucose levels, cells xe2x80x9cstarvexe2x80x9d since insulin-stimulated glucose entry into the cells is impaired. Triacylglycerol hydrolysis, fatty acid oxidation, glucogeogenesis, and ketone body formation are accelerated, which eventually causes a decrease in blood volume, and ultimately life-threatening situations.
In type-I diabetes, insulin is absent or nearly so because the pancreas lacks or has defective xcex2 cells. This condition results from an autoimmune response that selectively destroy the xcex2 cells. Individuals with insulin-dependent diabetes requires regular insulin injections to survive and must follow carefully balanced diet and exercise regimens.
Type II diabetes or non-insulin-dependent diabetes mellitus, accounts for over 90% of the diagnosed cases of diabetes and affects more than 16 million people in the U.S. and some 200 million people around the world. Yousef et al. (1999) Diabetes Review 7: 55-76. Contrasting with type I diabetes, type II diabetic individuals have normal or even greatly elevated insulin levels. Their symptoms arise from an apparent paucity of insulin receptors on normally insulin-responsive cells. It has been hypothesized that the increased insulin production resulting from overeating, consequently obesity, eventually, suppresses the synthesis of insulin receptor.
Type II diabetes causes various disabling microvascular complications in patients. Besides retinopathy, nephropathy, and neuropathy, the disease is also associated with accelerated atherosclerosis and premature cardiovascular morbidity and mortality. This increased incidence of atherosclerosis (e.g., myocardial infarction, stroke, and peripheral vascular disease) is intricately associated with insulin resistance, which is a major pathophysiologic abnormality in type II diabetes. The insulin resistance of type II diabetes contributes to the metabolic abnormalities of hyperglycemia, hyperinsulinemia, dyslipidemia, hypertension, and hypercoaglulation.
The genetic basis for obesity and diabetes has been gradually unveiled in recent years. Zhang et al. cloned the mouse obesity (ob) gene and its human homologue in 1994. Zhang et al. (1994) Nature 372:425-432. Mutation in ob leads to symptoms of obesity. The extensively-studies animal models for genetic obesity are mice which contain the autosomal recessive mutations ob/ob and db/db. These mutations are on chromosomes 6 and 4, respectively, but lead to clinically similar symptoms of obesity, including hyperphageria, severe abnormalities in glucose and insulin metabolism, very poor thermo-regulation and non-shivering thermogenesis, and extreme torpor and underdevelopment of the lean body mass. Restriction of the diet of these animals to restore a more normal body fat mass to lean body mass ration is fatal and does not result in a normal habitus.
The products of the ob and db genes constitute a hormone/receptor pair (leptin and the leptin receptor, respectively). The ob/ob and db/db mice are unable to produce (ob/ob) or respond to (db/db) leptin, a peptide hormone produced by fat cells. When leptin is administered to ob/ob mice, the mice eat less, become more active, and lose a significant amount of weight.
In addition to ob and db, several other single gene mutations resulting in obesity in mice have been identified. For example, the yellow mutation at the agouti locus has been found to cause a pleiotropic syndrome which causes moderate adult onset obesity, a yellow coat color, and a high incidence of tumor formation (Herberg and Coleman (1977) Metabolism 26:59), and an abnormal anatomic distribution of body fat (Coleman (1978) Diabetologia 14:141-148). Additionally, mutations at the fat and tubby loci cause moderately severe, maturity-onset obesity with somewhat milder abnormalities in glucose homeostasis than are observed in ob and db mice. Coleman and Eicher (1990) J. Heredity 81:424-427. Further, autosomal dominant mutations at the adipose locus of chromosome 7, have been shown to cause obesity.
Other animal models include fa/fa (fatty) rats, which bear many similarities to the ob/ob and db/db mice. One difference is that, while fa/fa rats are very sensitive to cold, their capacity for non-shivering thermogenesis is normal. Torpor seems to play a larger part in the maintenance of obesity in fa/fa rats than in the mice mutants. In addition, inbred mouse strains such as NZO mice and Japanese KK mice are moderately obese. Certain hybrid mice, such as the Wellesley mouse, become spontaneously fat. Further, several desert rodents, such as the spiny mouse, do not become obese in their natural habitats, but do become so when fed on standard laboratory feed.
Animals which have, been used as models for obesity have also been developed via physical or pharmacological methods. For example, bilateral lesions in the ventromedial hypothalamus (VMH) and ventrolateral hypothalamus (VLH) in the rat are associated, respectively, with hyperphagia and gross obesity and with aphagia and cachexia. Further, it has been demonstrated that feeding monosodium-glutamate (MSG) to new born mice also results in an obesity syndrome.
Attempts have been made to utilize such animal models in the study molecular causes of obesity. For example, adipsin, a murine serine protease with activity closely similar to human complement factor D, produced by adipocytes, has been found to be suppressed in ob/ob, db/db and MSG-induced obesity. Flier (1987) Science 237:405. The suppression of adipsin precedes the onset of obesity in each model. Lowell (1990) Endocrinology 126:1514. Further studies have mapped the locus of the defect in these models to activity of the adipsin promoter. Platt (1989) Proc. Natl. Acad. Sci. USA 86:7490. Further, alterations have been found in the expression of neuro-transmitter peptides in the hypothalamus of the ob/ob mouse (Wilding (1993) Endocrinology 132:1939), of glucose transporter proteins in islet xcex2-cells (Ohneda (1993) Diabetes 42:1065) and of the levels of G-proteins (McFarlane-Anderson (1992) Biochem. J. 282:15).
There still exists the need for improved treatment for obesity, diabetes and related diseases which have functional mechanisms different from those currently available.
The present invention relates to the discovery that certain cathepsins, particularly cathepsins L, K, and S, especially cathepsin L, play an important role in adipogenesis, a process of adipocyte or fat cell differentiation.
Leveraging the knowledge that the in vivo activity of fat regulating cathepsins, particularly cathepsins L, K, and S, especially cathepsin L, is tied to the regulation of fat storage, blood sugar levels and insulin levels, the present invention provides compositions, kits and methods for altering in vivo fat storage, blood sugar levels and/or insulin levels by altering the in vivo activity of fat regulating cathepsins. Such compositions, kits and methods may also be used to diagnose, monitor and treat various disease states which are related to improper, abnormal or undesirable fat storage levels, blood sugar levels and/or insulin levels.
In one embodiment, a method is provided for reducing fat storage in an animal comprising administering to the animal an agent which reduces an in vivo level of cathepsin L activity such that fat storage by the animal is reduced.
In another embodiment, a method is provided for reducing a blood sugar level of an animal comprising: administering to the animal an agent which reduces an in vivo level of cathepsin L activity such that the blood sugar level of the animal is reduced.
In yet another embodiment, a method is provided for reducing a blood insulin level of an animal comprising: administering to the animal an agent which reduces an in vivo level of cathepsin L activity such that the blood insulin level of the animal is reduced.
In yet another embodiment, a method is provided for treating an animal with one or more diseases selected from the group consisting of hyperinsulinmia, hyperglycermia, type II diabetes, hypertension, cardiovascular diseases, muscular dystrophy and infertility by administering to the animal an agent which reduces an in vivo level of cathepsin L activity.
These and other methods, compositions, and kits are described herein in greater detail.